PHP UserName and Password Validation Check

To begin with, let's assume that we have a PDO connection, just like you do already, for example with this function:

You can do something like:

// Usage:   $db = connectToDataBase($dbHost, $dbName, $dbUsername, $dbPassword);
// Pre:     $dbHost is the database hostname, 
//          $dbName is the name of the database itself,
//          $dbUsername is the username to access the database,
//          $dbPassword is the password for the user of the database.
// Post:    $db is an PDO connection to the database, based on the input parameters.
function connectToDataBase($dbHost, $dbName, $dbUsername, $dbPassword)
{
    try
    {
         return new PDO("mysql:host=$dbHost;dbname=$dbName;charset=UTF-8", $dbUsername, $dbPassword);
    }
    catch(Exception $PDOexception)
    {
        exit("<p>An error ocurred: Can't connect to database. </p><p>More preciesly: ". $PDOexception->getMessage(). "</p>");
    }
}

So that you can have a database connection like this:

$host = 'localhost';
$user = 'root';
$dataBaseName = 'databaseName';
$pass = '';

$db = connectToDataBase($host, $databaseName, $user, $pass);

So far we have the same stuff as you.

Now, I assume that we're on a PHP page where the user submitted his username and password, to begin with: check if we really received the username and the password, with the ternary oprator:

// receive parameters to log in with.
$userName = isset($_POST['userName']) ? $_POST['userName'] : false;
$password = isset($_POST['password']) ? $_POST['password'] : false;

Now you can validate if those inputs were actually posted:

// Check if all required parameters are set and make sure
// that a user is not logged in already

if(isset($_SESSION['loggedIn']))
{
    // You don't want an already logged in user to try to log in.
    $alrLogged = "You're already logged in.";
    $_SESSION['warningMessage'] = $alrLogged;
    header("Location: ../index.php");
}
else if($userName && $password)
{
    // Verify an user by the email address and password
    // submitted to this page
    verifyUser($userName, $password, $db);
}
else if($userName && (!($password)))
{
    $noPass = "You didn't fill out your password.";
    $_SESSION['warningMessage'] = $noPass;
    header("Location: ../index.php");
}
else if((!$userName) && $password)
{
    $noUserName = "You didn't fill out your user name.";
    $_SESSION['warningMessage'] = $noUserName;
    header("Location: ../index.php");
}
else if((!$userName) && (!($password)))
{
    $neither = "You didn't fill out your user name nor did you fill out your password.";
    $_SESSION['warningMessage'] = $neither;
    header("Location: ../index.php");
}
else
{
    $unknownError = "An unknown error occurred.". NL. "Try again or <a href='../sites/contact.php' title='Contact us' target='_blank'>contact us</a>.";
    $_SESSION['warningMessage'] = $unknownError;
    header("Location: ../index.php");
}

Now, let's assume that everything went well and you already have a database connection stored in the variable $db, then you can work with the function

verifyUser($userName, $password, $db);

Like already mentioned in the first else if statement:

// Usage:   verifyUser($userName, $password, $db);
// Pre:     $db has already been defined and is a reference
//          to a PDO connection.
//          $userName is of type string.
//          $password is of type string.
// Post:    $user exists and has been granted a session that declares
//          the fact that he is logged in.
function verifyUser($userName, $password, $db)
{
    $userExists = userExists($userName, $db); // Check if user exists with that username.
    if(!($user))
    {
        // User not found.
        // Create warning message.
        $notFound= "User not found.";
        $_SESSION['warningMessage'] = $notFound;
        header("Location: ../index.php");
    }
    else
    {
        // The user exists, here you can use your smart function which receives
        // the hash of the password of the user:
        $passwordHash = Password($UserName);
        // If you have PHPass, an awesome hashing library for PHP
        // http://www.openwall.com/phpass/
        // Then you can do this:
        $passwordMatch = PHPhassMatch($passwordHash , $password);
        // Or you can just create a basic functions which does the same;
        // Receive 1 parameter which is a hashed password, one which is not hashed,
        // so you hash the second one and check if the hashes match.

        if($passwordMatch)
        {
            // The user exists and he entered the correct password.
            $_SESSION['isLoggedIn'] = true;
            header("Location: ../index.php");
            // Whatever more you want to do.
        }
        else
        {
            // Password incorrect.
            // Create warning message.
            $wrongPass = "Username or password incorrect."; // Don't give to much info.
            $_SESSION['warningMessage'] = $wrongPass;
            header("Location: ../index.php");
        }
    }
}

And the function userExists($userName, $db) can be like:

function userExists($userName, $db)
{
    $stmt = $db->prepare("SELECT * FROM users WHERE USER_NAME = :USER_NAME;");
    $stmt->execute(array(":USER_NAME "=>$userName));
    $result = $stmt->fetch(PDO::FETCH_ASSOC);
    if($result)
    {
        // User exists.
        return true;
    }
    // User doesn't exist.
    return false;
}

Where the function Password is like:

function Password($UserName)
{
    $stmt = $db->prepare("Select USER_PASSWORD FROM user WHERE USER_NAME = :USER_NAME;");
    $stmt->execute(array(":USER_NAME"=>UserName));
    $result = $stmt->fetch(PDO::FETCH_ASSOC);
    if($result)
    {
        return $result['USER_PASSWORD'];
    }
    // No result.
    return false;
}

Again, make sure you're not matching plain text passwords, or basic shai1, md5 encryptiones etc. I really recommend that you take a look at PHPass.

I hope I'm making myself clear.